If the number of connections is high (400+) for some IP addresses, follow the steps from this KB article.

Cause

The MaxRequestWorkers limit has been reached and there are no free workers to serve new requests to Apache. As a result, requests are being served slowly or not served at all.

Resolution

Adjust the MaxRequestWorkers settings for Apache:

Note: The below values are just an example. They should be adjusted according to the server capabilities and load. The default MaxRequestWorkers value is 256. To increase it, you must also raise ServerLimit. Be careful, too high values of MaxRequestWorkers and ServerLimit directives may cause Apache to become unresponsive.

For CentOS/RHEL-based distribution

Find which Multi-Processing Module (MPM) is currently in use:

# httpd -V | grep MPM

Open the /etc/httpd/conf.modules.d/01-cgi.conf file in any text editor (For example, vi editor) and increase the values of the following directives (or add them if they are not present in the file):

Open the /etc/apache2/mods-enabled/mpm_event.conf in any text editor (For example, vi editor) and increase the values of the following directives (or add them if they are not present in the file):

CONFIG_TEXT: MaxRequestWorkers 400 ServerLimit 400

For MPM Prefork:

Open the /etc/apache2/mods-enabled/mpm_prefork.conf file in any text editor (For example, vi editor) and increase the values of the following directives (or add them if they are not present in the file):

CONFIG_TEXT: MaxRequestWorkers 400 ServerLimit 400

Restart Apache to apply the changes:

# service apache2 restart

Additional Information

Identifying a possible cause

We recommend to identify the cause of the MaxRequestWorkers limit being reached. Click on the "See how" section for details:

See how

Find websites that slow down Apache:

The other reason why the number of worker processes can increase is how long it takes to process a request.

@Dan, Am I right that even after changing 'MaxRequestWorkers' to 1000 and restarting apache you still see 'server reached MaxRequestWorkers setting' errors in apache logs? Additionally, you may check this article which contains troubleshooting steps.